Powder coatings are commonly applied to objects by powder spray guns that may be manually operated or automatic. In an automatic system, one or more spray guns are controlled to spray powder onto the objects as the objects are conveyed past the guns. In a manual gun operation, typically the object is suspended or otherwise positioned near a spray gun and the operator controls when the gun starts and stops spraying. A powder spray gun may be selected from a wide variety of gun designs. Since a spraying operation is intended to coat an object evenly, a common technique for spraying powder is to apply an electrostatic charge to the powder particles which causes the powder to better adhere to the object and also results in a more uniform application. Electrostatic spray guns include corona guns and tribocharging guns. In a corona type spray gun, a high voltage electrode is positioned in or near the powder flow path, either within the gun itself or just outside the gun near or at the gun nozzle. In a tribocharging type gun, the powder flow path through the gun body is made of suitable materials that impart an electrostatic charge to the powder as it is forced through the gun body.
The object being sprayed is electrically grounded such that the charged powder is attracted to and adheres to the object. This electrostatic attraction increases the transfer efficiency by increasing the amount of powder that adheres to the object. Transfer efficiency refers to the relationship between the amount of powder that adheres to the object being sprayed versus the amount of powder sprayed from the gun.
In most electrostatic spray systems, the powder is ejected from the gun nozzle as a cloud. This permits the powder spray to envelope the object to coat all the surfaces of the object, even when the object is irregular in geometric shape. Multiple guns may be positioned on different sides of the object and/or directed at different angles to increase the uniformity of the powder applied thereto. However, due to the inherent nature of the powder spray pattern, there is a substantial amount of powder that does not adhere to the object and ends up either falling to the floor or collecting on other objects and structures in the immediate area. This non-adherent powder residue is generally referred to as powder overspray.
Known powder spray systems utilize a source of powder that feeds powder to the spray guns. The supply system is commonly referred to as a powder feed center and may include a number of powder pumps that transfer powder from a feed hopper through a series of power hoses to the spray guns inside the spray booth. In general, an “application system” includes, as the powder flow path, at least spray gun, a powder source such as a feed hopper, a powder pump and a powder feed hose that connects the pump to the gun. In a known feed center, a suction tube or lance extends down into the feed hopper at one end and is connected to a powder pump at an opposite end. The pump draws powder from the hopper and the powder then flows from the pump through the powder feed hose to the spray gun. In such known systems, the powder flow path typically includes one or more turns, of about ninety degrees or so for example, and these non-straight paths can inhibit thorough cleaning during a color change operation. The known feed centers also require substantial time to purge and clean as part of a color change operation.
The presence of powder overspray necessarily dictates that there must be more powder passing through the spray system than is actually used to coat the target object. In other words, a substantial amount of powder is cycled through a spray system in the form powder that collects in the booth and in various filter and collection systems, and this amount of powder is far greater than the actual amount of powder that adheres to a target object. This excess powder is subject to contamination and in general adds to the problem of purging and cleaning the spray system in preparation for a color changeover.
Because powder overspray is generated during each spraying operation, spraying operations typically are performed within a spray booth. The spray booth is used for powder containment and may only be partially enclosed. Most spray booths have an air flow system that contains the powder overspray within the structure of the booth by producing a negative pressure zone that draws air from the powder booth along with powder overspray that is entrained in the air flow. The powder laden air is then transferred to a cartridge filter system or cyclone separator system outside the spray booth to recover the powder. However, in known spray booth systems, the powder overspray still tends to collect on the booth walls, ceiling and the booth floor. In electrostatic systems especially, the powder overspray will also tend to be attracted to and collect on any structure that is electrically grounded. The powder particles tend to be very small and well dispersed and therefore can collect in the smallest of recesses, seams and crevices and irregular spray booth wall structures.
Powder overspray presents a two-fold challenge. First, if possible it is usually desirable to try to reclaim or recover powder overspray so that the powder can be re-used during subsequent spraying operations. Known powder recovery systems typically work on the basis of a large air volume that entrains the powder overspray. These air flow volumes are routinely generated by conventional high volume exhaust fans. The powder laden air is then filtered, such as for example using cartridge type air filters or cyclone separators. The separated powder is then sieved to remove impurities and returned to a hopper or powder feed center where it is supplied once again to the spray guns. In known systems, the actual reintroduction of recovered powder to the powder spray application system is usually accomplished by a positive air pressure conveyance system back to a powder feed center through a series of hoses, valves and pumps. These additional components significantly increase the complexity of cleaning out the spray system for a color changeover.
Besides the challenge of recovering powder overspray for subsequent use or disposal, powder overspray that collects within the spray booth must be removed from the booth when changing over the powder coating color. In order to switch from one color to another the guns, booth and powder recovery system must be as completely purged of the previous colored powder as possible to prevent contamination of the subsequent colored powder. The operation of changing from one color to another is generally known as a “color change” operation and it is an ongoing challenge in the art to make spraying systems that are “quick color change” meaning that the goal is to keep reducing the down time when the spraying system is off line in order to clean the spraying apparatus and system. Thus, the amount of in-process powder, as well as the amount of powder overspray that remains in the spray booth, have a significant impact on the amount of time and effort it takes to perform a color change operation.
In known systems, a significant problem with cleanability and color change is that the powder, once it is sprayed from the guns, is not continuously recycled back to the feed center, but rather becomes resident at various stages within the spray system. In some systems for example, powder overspray may reside within the spray booth until a separate cleaning operation is performed after spraying is completed. Even in systems in which overspray is collected during a spraying operation, substantial amounts of powder can remain in the spray booth. Furthermore in some systems, powder overspray that is removed from the spray booth goes to a cyclone separator and falls into and resides in a cyclone bin until it is transferred to the feed center. The cyclone bin can be time consuming to clean. The transferred powder may then pass through a mini-cyclone in the feed center (because the powder from the cyclone is transferred under positive air pressure to the feed center and therefore is entrained in an air flow) before being dumped back into the feed hopper. Again, in this stage the powder may still reside in the mini-cyclone or sieve for a time before being returned to the feed hopper. If a cartridge filter system is used instead of a cyclone separator, the powder resides in the filters themselves until pulse cleaning is applied, and in any case the cartridge filters must be completely replaced during a color changeover.
Cyclones used in powder overspray recovery systems tend to be very large, particularly in terms of height. In some manufacturing plants, ceiling height may be limited or the customer may simply not want such a high structure for the cyclones. In such cases, shorter cyclones may be used. However, cyclone efficiency is related in part to the aspect ratio of the cyclone, namely the height to diameter ratio. The lower the aspect ratio the less efficient the cyclone for a given exhaust. However, some customers are willing to sacrifice efficiency for a smaller cyclone installation. This applies to single, twin or multiple cyclone arrangements preferably with single yield extraction configurations.
A problem with the powder overspray residing in various stages of the spray system is that the powder will tend to find even the smallest nook and cranny and even cake up, and substantial time will need to be spent cleaning this powder out.
Thus, color changeover typically includes having to clean powder from three major subsystems: the spray booth, the powder separator, and the feed center. Each subsystem has its own unique challenges to reducing the time it takes to completely clean out one powder color to prepare the system for spraying another color. During the cleaning time the spray system is completely down or off-line which represents lost time and increased costs, in addition to the costs associated with the labor needed to clean the various system components.
Cleaning a powder coating spray booth can be a labor-intensive effort. Powder coating materials, in varying degrees, tend to coat all the internal surfaces of the spray booth during a powder coating spray operation, which directly impacts color change time. In a production powder coating environment, minimizing the system down time to change from one color of powder coating material to another is a critical element in controlling operational costs. Seams between booth panels and recessed ledges, such as where access doors or automatic or manual spray application devices may be located, are typically hard to clean areas and tend to hold concentrations of oversprayed powder coating material that could present a contamination risk after a color change. In addition to seams and ledges and other recesses within the booth, charged powder can adhere to booth interior surfaces.
In typical powder coating booth construction, an outer steel framework is provided for supporting individual panel members which form the roof, side and end walls of the booth. These panel members are known to be made of a fabricated or thermoformed plastic, such as polypropylene, polyvinyl chloride (PVC), polyvinyl carbonate or polycarbonate. The floor may also be of thermoformed plastic or stainless steel construction. In other known embodiments, powder coating spray booths can have metallic walls, ceilings and vestibule ends, as well a metallic floor and exterior support framework.
U.S. Pat. No. 5,833,751 to Tucker is an example of a powder coating spray booth intended to reduce powder particle adhesion to the interior surfaces of the booth during an electrostatic powder spray operation. Tucker discloses a booth chamber comprising a pair of thermoformed plastic shells with smooth curvilinear interior surfaces that are intended to inhibit oversprayed powder particle adhesion. Two identical ends connect with the shells and an external support frame is disclosed, but not shown. Possible booth materials disclosed include polycarbonate.
Known booth materials are available in limited sizes requiring some method of seaming to generate the overall size. These seams require much effort and cost to achieve a virtually uninterrupted, seamless surface.
In addition, known powder coating spray booths have numerous features that reduce operational efficiencies. These sub-optimal features are evidenced during powder coating color changes between successive runs of different coating colors and during assembly and maintenance of the booth itself. Known powder coating spray booths use metallic external support frames and stainless steel or thermoplastic, floors, walls and ceilings. During an electrostatic powder spray coating operation, oversprayed powder material can actually be attracted and adhere to these booth interior surfaces. Higher concentrations of oversprayed powder coating material are typically seen in the immediate vicinity of the highly conductive steel frame members, which are typically grounded. Although thermoformed plastics are typically thought of as insulators, their insulation properties vary and powder particle adhesion can vary with the conductance and resistance of these materials. With age, physical properties of the thermoformed plastic materials can change with corresponding increases in powder particle adhesion, as they can absorb moisture from the ambient air over time. Ultraviolet light is also known to change the physical properties of thermoplastics over time.
In addition, typical booths have numerous design features that act to increase accumulated oversprayed powder coating materials in the spray booth, thus increasing cleaning times during color change operations. In booths using panel members connected with each other and supported by an external frame, numerous seams exist throughout the booth interior that entrap oversprayed powder coating material, thereby making the booth harder to clean during a color change or routine booth maintenance. In addition to the seams, ledges are present in some powder coating spray booths on which spray gun application devices rest and are mounted, and where openings for doors and other access portals are reinforced and secured, for example. These ledges can either extend into the booth or, more typically, extend away from the inner surface of the booth. Even if otherwise angled or curved toward the floor from the typically vertical side walls, oversprayed powder coating material still tends to accumulate in these areas, thus making them more difficult to clean, as well.
Known prior systems for removing powder overspray from a spray booth include active systems in which floor sweepers and other mechanical devices are used to mechanically contact the powder and push it off the floor into a receiving device. These systems however tend to be cumbersome and are not thorough in the amount of powder removed from the booth. A substantial effort by one or more operators is still required to completely remove powder from the booth. Thus there can be a large amount of in-process powder and powder overspray on the booth structure.
In passive removal systems, powder is removed from the floor in a non-contact manner. In one known system, a rectangular floor in the form of a continuous linearly moving belt transports powder over to a collection device such as a vacuum system that removes powder from the belt. Such systems are very complicated mechanically and do not do an adequate job in removing powder from the belt, so much so that in some cases a color change requires a change of the belt itself.
It is desired therefore to provide a spray booth that is easy to clean as part of a color change operation and operates so as to minimize the amount of in-process powder and the amount of powder overspray remaining in the spray booth after a spraying operation is completed.
It is further desired to provide a powder coating spray system and associated subsystems including a powder recovery system that substantially reduce the residence time of powder overspray within the system between the spray gun nozzle and the feed hopper. The spray system should remove as much powder overspray as possible from the spray booth and transfer it back to the feed center during a spraying operation. Thus the amount of residual powder overspray needing to be manually cleaned from the subsystems will be largely eliminated. It is further desired to provide a powder feed center that is easier and faster to clean as part of a color change operation.
It is further desired to provide method and apparatus for influencing the efficiency of a cyclone separator used with a powder coating spraying system.